The National Highway Traffic Safety Administration's (NHTSA) Federal Motor Vehicle Safety Standard 201 (FMVSS201), entitled “Occupant Protection in Interior Impact,” provides specific requirements for upper interior impact protection of an occupant of a motor vehicle. The NHTSA estimates that even with airbags, the vehicle occupant's impact with the pillars, roof-side rails, windshield header, and rear header result in 1,591 annual passenger car occupant fatalities and 575 annual LTV occupant fatalities. Further, the NHTSA estimates that such head impacts result in nearly 13,600 moderate to critical (but not fatal) passenger car occupant injuries and more than 5,200 serious LTV occupant injuries. According to FMVSS201, manufacturers decide how to meet the requirements of the Aug. 18, 1995 final rule. A popular method of compliance is the installation of energy-absorbing countermeasures (CM) such as padding, which manufacturers hope will reduce the number and severity of these injuries. The NHTSA estimates that the new requirements will prevent 675 to 768 moderate to critical passenger car occupant injuries and 873 to 1,045 fatalities. Further development of dynamic head-protection countermeasures offer the potential for further reducing injuries.
While impact-zone designers typically understand head injury criteria and the various safety standards, a major problem with current design of countermeasures is that testing has not been repeatable or consistent from one laboratory to the next. While careful attention to testing details has removed most of the ambiguity that plagued past design efforts, particular attention must be paid to the vehicle being impacted, including whether the doors are shut and the windows rolled down, and whether the same target spot on the free motion headform is used, and whether the launch angles and actual launch speed are the same.
Finite Element Analysis (FEA) is one proposed solution, but it is effective only approximately half of the time because of complex failure modes and limited high-speed material data. Once a material is selected, material models may be created, but that requires additional research effort itself. While FEA is a valuable tool for enhancing countermeasure design for a given vehicle, it cannot solve a design problem nor give an understanding to the physical limits of possibility. FEA is limited in predicting vehicle FMH responses because:                Materials are often utilized well into the nonlinear region of the stress strain responses.        Contact elements are typically required so those components can push on each other but not necessarily pull.        Many countermeasures accomplish energy absorption through a fracturing mechanism that is difficult to model.        Many of the countermeasure materials exhibit a speed dependent response, which may not be available in material data files for FEA.        The materials (including steel) typically behave as a spring damper combination and not simply as a nonlinear spring.        
Thus, in order to use FEA to accurately predict the degree of injury, a material model should include speed, damping, temperature, nonlinear and fracture components built into its code to accurately model three-dimensional sheet metal, headliner and plastic components. Of course, if enough effort and research is put into understanding specific materials, then FEA could be used with limited success. Therefore, an impediment to practical application of the FEA is the substantial variety of materials in use today.
Most current attempts to pass the FMVSS201 focus on meeting the safety standards through a trial and error approach. The problem with this approach is that while adequate space may exist in a particular design to accommodate a countermeasure, the countermeasure may be too soft or too hard. Both scenarios result in a high head injury factor, possibly even the same number, and both tests would be considered failures. Failure to understand this and thus pursuing parallel path solutions and alternatives has resulted in substantial expense to the automobile industry.